Nitrite esters, i.e., esters of nitrous acid, are generally colorless liquids which have found use in areas such as additives to motor fuels, stabilizers for vinyl compounds, as spasmolytic agents, as reagents for diazotization and as reagents for chemical synthesis.
The classical method for preparing nitrite esters involves the liquid phase reaction of sodium nitrite and sulfuric acid with a desired alcohol. The reaction is normally carried out at ice temperatures, due to the extremely exothermic nature of the reaction, to form nitrite esters as follows: EQU 2NaNO.sub.2 +H.sub.2 SO.sub.4 +2ROH.fwdarw.2RONO+Na.sub.2 SO.sub.4 +2H.sub.2 O
The nitrite ester formed is insoluble in water (less than about 1 percent in water or water in the nitrite ester) so that the nitrite ester is easily separated from the reaction products.
The production of nitrite esters in the liquid phase is disclosed in U.S. Pat. No. 2,166,698 wherein nitrite esters are produced by reacting an appreciably water soluble open-chain saturated aliphatic compound containing a plurality of esterifiable carbinol groups with nitrous acid in an aqueous medium and removing a nitrite ester from the reaction system substantially as soon as it is formed therein. The nitrite esters formed therein react rapidly with alcohol by ester interchange, e.g., ethyl alcohol, to form an alkyl nitrite, e.g., ethyl nitrite.
U.S. Pat. No. 2,739,166 describes producing alkyl nitrites in a liquid phase process by bubbling nitrogen dioxide gas into a cooled liquid monohydric aliphatic alcohol.
In British Patent Specification No. 586,022, a liquid phase process is disclosed for the preparation of nitric acid esters which comprises reacting an alcohol with nitrogen tetroxide in the liquid phase.
German Patentschrift No. 1,156,775 discloses a liquid phase process for preparing esters of nitrous acid by continuously removing the formed ester by employing alcohol in molar excess over dinitrogen trioxide at temperatures below the boiling point of the alcohol and simultaneously distilling off the ester formed. In addition, the reference acknowledges that the vapor phase decomposition of alcohols with nitrogen dioxide-nitrogen monoxide mixtures at temperatures between 100.degree. and 420.degree. is known.
Japanese Application No. 53-8268/78 describes the preparation of nitrite esters by a conventional liquid phase process as part of the continuous production of oxalic acid diester using nitrite ester as a starting material. The nitrous acid ester in the process if formed by employing a common gas-liquid contacting apparatus to react nitrogen oxides with an alcohol at a temperature lower than the boiling point of the alcohol.
The aforementioned processes are to be distinguished from a vapor phase process in that in liquid phase processes the separation of the nitrite ester product is difficult and oxidation of alcohol in the liquid phase during the manufacture or separation can occur to form unwanted by-products. In addition, the separation of the highly flammable and toxic nitrite ester from the liquid phase can prove to be a major safety and health problem.
A vapor phase process is disclosed in U.S. Pat. No. 2,831,822. This patent discloses a process for the preparation of nitrite esters which comprises reacting a vaporized alcohol with from 0.4 to 0.6 mole of nitrogen dioxide and 0.4 to 2.0 moles of nitric oxide per mole of alcohol in the presence of from 2 to 25 moles of diluent which may be water, nitrogen, or carbon dioxide, at a temperature between 100.degree. C. and 420.degree. C. with a contact time of 1-10 seconds.
Table 1 of U.S. Pat. No. 2,831,882 describes 4 examples wherein the molar ratio of NO to NO.sub.2 is greater than one but wherein the molar ratio of alcohol to combined NO and NO.sub.2 needed to react with all the N.sub.2 O.sub.3 possible, is less than 1. In each case, in order to achieve a relatively high conversion, above 80%, it was necessary to employ temperatures in excess of about 130.degree. C. In addition to the increased rate of decomposition of nitrite ester (product) at these temperatures, the reference creates additional problems by requiring the employment in each example of a significant amount of water. The use of water in the process results in the formation of nitric acid at least some of which will be present in the ester product.
Table II of U.S. Pat. No. 2,831,882 describes examples which employ various molar ratios of nitric acid, nitrogen dioxide, nitric oxide and nitrogen dioxide or nitric acid to alcohol (n-butanol). In each example the molar ratio of alcohol to total nitrogen oxides is less than one. Further, in each example a temperature in excess of 170.degree. C. was required to provide a conversion to nitrite ester product greater than 70 percent. In addition, the patent, at column 3, lines 55 to 64, states that:
"When nitrogen dioxide is reacted with the alcohol in the presence of water at temperatures below 250.degree. C., equimolar proportions of the nitrite ester and nitric acid are formed. By increasing the temperature of the reaction to 350.degree. C., the formation of nitric acid was almost eliminated, and the conversion to nitrite ester increased. These results are consistent with the previously mentioned mechanism of reaction, since a higher temperature increases the decomposition of both nitric acid and nitrogen dioxide."
Thus, not only does the process require relatively high temperatures but also results in the formation of nitric acid which may be decomposed at higher temperatures.
Examle 1 of U.S. Pat. No. 2,831,882 prepares the nitrite of isopropyl alcohol. This example in U.S. Pat. No. 2,831,882 provides a molar ratio of NO to NO.sub.2 of less than one and an isopropanol to a combined NO and NO.sub.2 molar ratio of greater than one. The process is operated at a pressure of 90 psi with only a 58 percent conversion to product (the reference reports a yield of 89% based, presumably, on nitrite ester converted from the alcohol consumed which in reality is a conversion of about 39 percent based on the nitric oxide and nitrogen dioxide available). In addition, the process results in the incomplete reaction of the nitrogen dioxide. The unreacted nitrogen dioxide may be quite deleterious to any further process in which the nitrite ester is employed.
U.S. Pat. No. 4,229,591 uses the preparation of nitrite esters as an intermediate step in a process for preparing a diester of oxalic acid. The patent discloses, at Col. 2, lines 21-35, that:
The nitrogen compound used in the present process need not necessarily be in the form of an ester of nitrous acid, and a compound which forms an ester of nitrous acid in the reaction system may also be used. It may also be advantageous to use an alcohol along with a nitrogen compound selected from the group consisting of nitrogen monoxide, nitrogen dioxide, dinitrogen trioxide and dinitrogen tetroxide, and hydrates of a nitrogen oxide instead of an ester of nitrous acid by introducing a gas containing molecular oxygen into the system in cases where nitrogen monoxide is used. As the hydrates of a nitrogen oxide may effectively be used nitric acid, nitrous acid and the like. An alcohol to be used in such cases is selected from alcohols which constitute esters of nitrous acid as mentioned hereinbelow.
To overcome the problems associated with the known processes for the preparation of nitrite esters a process must be found that may be run in the vapor phase at relatively low temperatures and pressures while minimizing the formation of by-products.
The aforementioned processes fail to appreciate the need to provide a vapor phase process wherein the molar ratio of nitric oxide (NO) to nitrogen dioxide (NO.sub.2) and the molar ratio of alcohol to the combined molar quantity of nitric oxide and nitrogen dioxide are each greater than one in order to effect an efficient process, one which can run at relatively low temperatures and pressures while minimizing the formation of by-products.